In time-of-flight (TOF) mass spectrometers, charged particles are accelerated along a flight path by the application of an electric potential and mass-to-charge ratios (m/z) are determined by measuring time of flight over a predetermined distance using a detection arrangement. When choosing a detector arrangement, considerations may include: the response time of the detector; the detector dynamic range; the smallest detectable signal (detection limit); the ability to detect multiple charged particles arriving at the detector at the same time; and the time resolution of the detector, which is its ability to differentiate between particles arriving at the detector at different times.
The time taken by a charged particle to reach a given point or plane depends on its initial kinetic energy, its m/z ratio and the length of the flight path. Orthogonal TOF mass spectrometers typically have a relatively short flight path. Therefore, particles of different m/z ratio will not have a significant difference in their time of flights, and so these mass spectrometers are limited in their mass resolution even for well-defined ion beams and with fast acquisition systems. A useful high dynamic range is achieved in these TOF spectrometers by the summation of a great many spectra, each spectrum typically containing tens to hundreds detected ions. In addition, detectors with several anodes could be employed, each anode having an individual output.
The length of the flight path may be increased without significantly increasing the size of the instrument by causing the charged particle beam to be reflected multiple times thus folding ion trajectories within a limited volume. This is achieved by using multiple electrostatic ion mirrors, or multiple electrostatic sectors, or any combination of the above. In many cases, multiple mirrors or sectors could be replaced by an integrated construction extended along a direction substantially orthogonal to the direction of time-of-flight separation. The extent to which this increase in the length of flight path is desirable depends on the capabilities of the detection arrangement.
All these systems are characterised by a multitude of segments, each segment having a region of ion acceleration, (i.e. reflection or deflection region) followed by a region where such acceleration is relatively small (i.e. substantially field-free region). Here and below, all such systems will be referred as multi-reflection TOF.
From an ion optical point of view multi-reflection TOFs are a sub-class of a more general class of electrostatic traps, and could be subdivided into “open type” and “closed type” multi-reflection TOFs. “Open type” relates to systems where ion trajectories can not be confined within the trap for an indefinite time but only for a limited number of reflections. Typically the ion path is not reflected onto itself. Such systems do not suffer from limitations of mass range typical for “closed-type” electrostatic traps where ions are forced to follow substantially the same path and therefore different regions of m/z range increasingly overlap.
The main advantage of multi-reflection TOF mass spectrometers is the increase of the length of the flight path and thereby of the time-of-flight. Hence the difference in time of flight between particles of different m/z ratios (i.e. TOF dispersion) is increased, thus improving the mass resolution. At the same time, as the time of flight is increased, the repetition rate is reduced. The reduced repetition rate reduces the number of spectra that can be summed and therefore limits the dynamic range the spectrometer can achieve, in a given time period.
The duty cycle of analysis is also reduced but it could be restored by using ion storage devices for accumulating ions between injections into TOF. However, use of ion storage devices to preserve duty cycle increases the number of ions in each mass peak thus increasing the range of intensities in a single shot beyond capabilities of known detectors.
Hence, existing TOF instruments are unable to provide high mass resolution together with high dynamic range. They are therefore unable to differentiate between one type of particle, with a first m/z ratio, in high abundance in a charged particle beam, and a second type of particle, with an m/z ratio close to the first m/z ratio, but in low abundance in the beam.